Many enzymes require metal ions as participants in their catalytic mechanisms. Although significant advances have been made in characterizing the nature of metal-protein interactions, very little progress has been made toward understanding the roles which metal ions play in the detailed mechanisms of enzymatic reactions. Our approach to the study of metal requiring enzymes involves the application of stopped flow-temperature jump relaxation spectrometry. This technique permits us to investigate certain discrete steps of enzymatic processes, such as enzyme-substrate complex formation, enzyme or complex isomerizations, interconversion of intermediates, and enzyme- product complex breakdown. The kinetics of each of the observed steps in an enzymatic reaction will be investigated for each of a series of metal ions having diversified properties of charge, size, stereochemical preferences, polarizabilities, oxidation-reduction potentials, etc. From a correlation of such properties of each metal ion with its kinetic effects on the individual reaction steps, we should be able to develop a more detailed picture of the function of the metal ion in the activation process. This procedure will be applied to the study of the metalloenzymes carbonic anhydrase and xanthine oxidase.